Embodiments of the invention relate generally to compressed air storage systems and, more particularly, to a system and method of using a compressed air storage system with a gas turbine.
Types of compressed air energy storage systems include diabatic compressed air energy storage (diabatic-CAES) systems and adiabatic compressed air energy storage (ACAES) systems. Such systems typically include a compression train having one or more compressors that operate during a compression stage to compress intake air to 80 bars or more, where the energy stored is available to later power a turbine to generate electricity. Typically, the compressed air can be stored in several types of underground media that include but are not limited to porous rock formations, depleted natural gas/oil fields, and caverns in salt or rock formations. Alternatively, the compressed air can be stored in above-ground systems such as, for example, high pressure pipelines similar to that used for conveying natural gas. However, above-ground systems tend to be expensive and typically do not have a storage capacity comparable to an underground cavern—though they can be attractive in that they can be sited in areas where underground formations are not available.
A diabatic-CAES system typically loses a heat of compression of the air to an ambient environment, while an adiabatic system stores the heat of compression for later use. During operation of the compression stage, compressed air typically exits the compressor having an elevated temperature of, for instance, between 550° C. and 650° C., which is due in large part to heat of compression of the air. The amount of energy contained therein is a function of at least its temperature difference with ambient, its pressure (i.e., a total mass of air), and its heat capacity. Thus, in a diabatic-CAES system, although the heat of compression may be largely present when entering the cavern, its energetic value and availability is diminished as it mixes with the cavern air and as it further cools to surrounding or ambient temperature during storage—leading to a low overall efficiency.
ACAES systems, on the other hand, improve system efficiency by capturing and storing the heat of compression for later use. In such a system, a thermal energy storage (TES) unit is positioned between the compressor and the cavern. Typically, a TES includes a medium for heat storage, and hot air from the compression stage is passed therethrough, transferring its heat of compression to the medium in the process. Some systems include air that exits the TES at or near ambient temperature, thus the TES is able to store heat that is due to compression, as compared to a diabatic system. As such, the air may enter the cavern at or near ambient temperature but at high pressure, and little energy is lost due to any temperature difference between the compressed air and ambient temperature. In order to store the large amounts of energy from the heat of compression, the medium of the TES typically includes a high heat capacity material. For instance, a TES may include concrete, stone, a fluid such as oil, a molten salt, or a phase-change material. The energy stored in the TES is then available to heat the stored high-pressure air as it is drawn therefrom during an energy generation cycle. Thus, ACAES systems provide improved efficiency over diabatic systems, some systems reaching to 75% first law efficiency or greater.
However, overall system efficiency is not necessarily the guiding parameter for determining whether to build or operate a compressed air energy storage system. System efficiency is an important consideration, but there are other factors to be considered, as well. For instance, compressed air storage systems typically derive power from the electric grid during, for instance, relatively less-expensive, off-peak, or low-demand hours such as at night. Alternatively, energy storage operation may derive power from renewable sources such as wind, which may provide intermittent power that may be during less desirable low-demand evening or nighttime hours. The compressed air is then later available to drive one or more turbines to produce electrical energy during an energy generation stage, which may be during peak-power needs. As such, compressed air energy storage systems enable electricity suppliers to store relatively low-cost energy that may then be produced during peak demand periods, which may be sold at a premium. And, another factor to consider in whether to build a compressed air energy storage system is capital expenditure. Because of an increased power surge capability provided by such a compressed air storage system, that capability may obviate the need to build additional and expensive conventional power generation capacity such as natural gas or coal-fired power plants.
Thus, the decision whether to build a compressed air energy storage system is influenced by many factors, that include but are not limited to cost of system operation, availability of renewable energy sources, total power capacity in a given market and price swings between low-demand and high-demand periods, and compressed air energy storage system efficiency. However, construction of such standalone compressed air energy storage systems itself typically includes an initial capital expenditure that may not be justified, despite the ability to collect and store low-cost energy and sell the stored energy during times of peak demand. Thus, in some markets where there is a need for power surge capability, where there is surplus evening capacity, or where renewable energy sources are available, power producers may nevertheless elect not to build such a system because the initial costs of construction are too high.
Gas turbines typically include one or more air compressors coupled together and configured to output compressed air to a combustor. The combustor is typically powered by natural gas and combusts the natural gas therein by using the compressed air from the one or more air compressors. A first turbine is powered by exhaust products of the combustor, and the first turbine is typically mechanically coupled to the one or more compressors to provide mechanical power thereto. The exhaust products exit the first turbine and power a second, or power turbine, which is itself typically coupled to a generator to produce electricity. As such, the combustor provides power to the first turbine, powering the one or more compressors, and the combustor also provides power to the second or power turbine to generate electrical power therefrom.
However, such systems cannot typically benefit from or consume renewable energy resources and do not typically have an energy storage capability, aside from their primary power supply of natural gas. Thus, to meet total power capacity demands of a given area or market, and because of the aforementioned high cost of constructing a compressed air energy storage system, power providers may elect to construct power systems that are based only on natural gas-powered systems. This election precludes power providers from obtaining the benefit of energy storage and from obtaining the benefit of renewable energy systems such as wind power. As such, in markets where renewable energy sources may be readily available, or where evening power may be significantly less expensive to produce than peak power, power providers may nevertheless elect to avoid an initial cost of construction and construct only a natural gas powered turbine system for electrical power generation. Thus, because of initial high cost of construction, power providers may be failing to take advantage of a renewable source that is readily available.
Therefore, it would be desirable to design an apparatus and develop a method of construction that reduces an initial cost of system construction that overcomes the aforementioned drawbacks.